Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtgui/history.cc

@@ -25,7 +25,7 @@ using namespace rtengine::procparams;
 
 Glib::ustring eventDescrArray[NUMOFEVENTS];
 
-History::History (bool bookmarkSupport) : blistener(NULL), tpc (NULL), bmnum (1)
+History::History (bool bookmarkSupport) : blistener(nullptr), tpc (nullptr), bmnum (1)
 {
 
     blistenerLock = false; // sets default that the Before preview will not be locked